1. Field of the Invention
This invention relates to preparation of composite plastics and more particularly, to directly moldable resin compositions adapted for use in ordinary molding machines.
2. Description of the Prior Art
Preparation of dispersion composite plastics has ordinarily been performed by the following three methods.
1. Compounding
Various starting materials such as synthetic resins, fillers and additives are provided in predetermined amounts and kneaded under melting conditions of the resins in a kneader and granulator machine, and pelletized into a desired form. This compounding technique is the most widely employed method of preparing composite plastics.
2. Concentrate or Master Batch Method
Compounding ingredients other than synthetic resins such as fillers, additives and the like are admixed with a synthetic resin to be mixed in concentrations higher by several times than in intended concentrations, and kneaded and pelletized in a kneader and granulator machine.
Upon application, the resulting pellets are mixed or diluted with a synthetic resin of the same type as used above to have the intended concentrations of the compounding ingredients. This method has widely been used on mixing of colorants with synthetic resins.
3. Direct Method
Synthetic resins and compounding ingredients such as fillers, additives and the like are charged into the feed port of a molding machine as they are without melting and kneading, and are plasticized, kneaded and then molded in a screw cylinder. This technique is now adoptged for use in extrusion molding, injection molding and the like, and has extensively been studied and performed as useful especially in obtaining moldings of glass fiber-incorporated composite plastics.
These techniques have the following advantages, respectively. That is, the compounding method is advantageous in that starting materials are formulated to have a composition required for an intended molded article, and can thus be molded with ease without needing additional steps. The master batch method is low in production cost per unit weight of molding materials. Moreover, the direct method has the advantage that because the kneading and molding steps are effected at the same time and thus the power energy as will usually be needed in the needing step can be saved, making it possible to reduce the production cost, coupled with another advantage that because of the reduction in heat history, the obtained moldings exhibit excellent physical strengths and stable quality.
In order to obtain composite materials of the uniform quality, it is necessary to impart a great volumetric change and a strong shearing force to starting materials at the time of kneading. Accordingly, kneading conditions such as kneading temperature, the number of revolutions of screw, kneading temperature and the like, and the design in shape of the screw give a great influence on the quality of composite material. In general, the screw and the cylinder of an ordinary injection molding machine are very simple in shape and have the ability to plasticize resins but are not designed to disperse fillers in resins by kneading. Accordingly, the kneading in injection molding machines has heretofore been attempted only experimentally but has not been put into practice widely.
In recent years, a screw and cylinder arrangement adapted for the injection molding by the direct method are commercially sold. This screw and cylinder arrangement has the functions of plasticization, kneading and injection molding but is of the specific and very complicated form and is thus very expensive. Additionally, the difficulty encounters in changing color or materials.